The polyols employed as components for formulations for polyurethanes generally consist in products obtained from the polycondensation of ethylene oxide, propylene oxide and respective glycols, in the presence of polyhydroxyl compounds, such as for example as glycerin, trimethylolpropane, pentaerythritol, sorbitol, or of polyfunctional alkanolamines, such as for example as ethylene diamines, triethanolamine and the like.
These polyols have rather high molecular weights, generally comprised between 300 and 6000.
Such compounds, due to reaction with polyisocyanates in the presence of suitable catalysts, lead either to polyurethanes or to polyurethanes-isocyanurates, depending on the equivalent ratio (NCO/OH) and on the weight ratio.
A drawback common to these polyols consists in that their polymeric chains exhibit a certain number of C--O--C bonds (ethereal bonds), that are very sensitive to the action of heat and of flame which results in a significant decrease in the flame and heat resistance characteristics of the polyurethanes obtained by use of such polyols.
On the other hand, if diols or polyols are used containing no or few ethereal bonds, it is necessary to employ compounds having a relatively low molecular weight, generally lower than 300, in order to reduce the amount of diols or of polyols in respect to the polyisocyanates. In this case, however, serious difficulties are met because these products are too reactive and have too low a viscosity to be capable of providing polyurethane foams having acceptable mechanical properties (especially friability).